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Date: Mon, 4 Sep 2017 20:29:32 +0100
From: Ard Biesheuvel <>
To: Nicolas Pitre <>
Cc: "" <>, 
	Kernel Hardening <>, Arnd Bergmann <>, 
	Russell King <>, Kees Cook <>, 
	Thomas Garnier <>, Marc Zyngier <>, 
	Mark Rutland <>, Tony Lindgren <>, 
	Matt Fleming <>, Dave Martin <>
Subject: Re: [PATCH v2 24/29] ARM: kernel: implement randomization of the
 kernel load address

On 4 September 2017 at 19:44, Nicolas Pitre <> wrote:
> On Sun, 3 Sep 2017, Ard Biesheuvel wrote:
>> This implements randomization of the placement of the kernel image
>> inside the lowmem region. It is intended to work together with the
>> decompressor to place the kernel at an offset in physical memory
>> that is a multiple of 2 MB, and to take the same offset into account
>> when creating the virtual mapping.
>> This uses runtime relocation of the kernel built as a PIE binary, to
>> fix up all absolute symbol references to refer to their runtime virtual
>> address. The physical-to-virtual mapping remains unchanged.
>> In order to allow the decompressor to hand over to the core kernel
>> without making assumptions that are not guaranteed to hold when
>> invoking the core kernel directly using bootloaders that are not
>> KASLR aware, the KASLR offset is expected to be placed in r3 when
>> entering the kernel 4 bytes past the entry point, skipping the first
>> instruction.
>> Cc: Russell King <>
>> Signed-off-by: Ard Biesheuvel <>
>> ---
>>  arch/arm/Kconfig       |  15 +++
>>  arch/arm/kernel/head.S | 103 ++++++++++++++++++--
>>  2 files changed, 109 insertions(+), 9 deletions(-)
>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>> index 300add3b8023..fe4a2cd1f15c 100644
>> --- a/arch/arm/Kconfig
>> +++ b/arch/arm/Kconfig
>> @@ -1825,6 +1825,21 @@ config XEN
>>       help
>>         Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
>> +     bool "Randomize the address of the kernel image"
>> +     depends on MMU && AUTO_ZRELADDR
>> +     depends on !XIP_KERNEL && !ZBOOT_ROM
> This should work even if ZBOOT_ROM is selected. The ZBOOT_ROM option
> allows for the decompressor executing directly from ROM and
> decompressing the kernel anywhere in RAM.

In principle, yes. But currently, the code is not entirely XIP clean,
i.e., the kaslr_offset and __efi_boot variables are in .text but
assumed to be writable.

For UEFI, this does not really matter, because that implies !XIP.
However, I agree it would be nice if ZBOOT_ROM could run this code as
well, but it requires a global variable. I will try to make that work.

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